The present invention relates to improvements in developing devices, and particularly relates to improvements in developing devices in which developer is smoothly conveyed, uneven image density does not occur, and further developer does not scatter and leak from the container of the developing device.
Regarding different forms of image recording apparatus, the following are widely known.
(a) An electrophotographic image recording apparatus, which is called an electrophotographic copier.
(b) An image recording apparatus such as a laser beam printer, which is characterized in that: a surface is divided into a plurality of small areas, which will be called dots hereinafter; digital information which indicates coloring or non-coloring, is given to each dot; and an image is formed on the above-described surface according to the digital information.
In these kinds of image recording apparatus, a toner supply means which supplies a coloring agent (which will be called toner hereinafter), is needed on the assumption that an image signal medium, for example an electrophotographic photoreceptor drum on which a latent image is formed, wherein a coloring command is given to the dot on the image signal medium on which coloring is to be conducted and a non-coloring command is given to the dot on which coloring is not to be conducted.
Regarding the toner supply means, toner must be uniformly and detachably attracted to the surface of the toner supply means.
The present invention relates to improvements in this kind of toner supply means, which will be called a developing means in this specification hereinafter.
Referring to FIG. 2a, a conventional developing unit will be explained as follows.
In FIG. 2a, the numeral (1) is a developer stirring means which is composed of two screws (11) and (12). Toner is supplied to the developer stirring means (1) from a toner casing means (not shown in the drawing) which is located above the developer stirring means (1). The supplied toner is conveyed by the screw (11) to the viewer's side in the drawing. At this moment, in this example, a coloring material carrying means (which will be called a carrier hereinafter), which is composed of particles, the diameter of which is approximately 50 .mu.m, made from magnetic material, is supplied from the side of the screw (12). As a result, the toner and carrier are uniformly stirred and mixed so that they are transformed into a developer.
This developer is uniformly contacted with the surface of the developer conveying means (7) (which is a triangular body of rotation in this example) with pressure by the action of the screw (12), wherein the developer conveying means (7) functions in the subsequent process.
The numeral (3) is a developer holding means which is composed of the sleeve (31), a cylindrical body of rotation, and a magnetic means (32) which is fixed to the inside of the sleeve (31). The surface of the sleeve (31) attracts developer. When the sleeve (31) is rotated and contacted with an image signal medium (for example, an electrostatic photoreceptor drum on which a latent image is formed), which is not illustrated in the drawing, the toner contained in the developer attracted onto the surface of the sleeve (31) is supplied from the sleeve (31) to the latent image formed on the above-described image signal medium so that the above-described latent image can be developed. In the way described above, only carrier is left on the region of the sleeve (31) surface from which the toner was supplied to the latent image, so that the toner concentration on the sleeve (31) becomes uneven.
The function of the above-described magnetic means is as follows: the magnetic means attracts the developer onto the above-described sleeve (31) while development is conducted; and the means removes the residual toner from the above-described sleeve (31) after development has been completed.
The numeral (4) is a developer layer thickness regulating means which controls the developer so that a desirable thickness of developer layer can be formed on the above-described sleeve (31).
The numeral (7) is a developer conveying means which conveys the developer pushed out by the screw (12) to the developer holding means (3), and which conveys the developer (in which the amount of toner is small and the amount of carrier is relatively large) that has been removed from the developer holding means (3) to the region of the developer stirring means (1).
The numeral (9) is a developing unit body in which the above-described units are installed.
The composition of a conventional developing unit has been explained above. On the other hand, the following idea is proposed with regard to the means which conveys the developer removed from the sleeve (31) to the region of the stirring screw (12): for example, a magnetic conveying means composed of a cylindrical rotative body on which a magnetic pole is formed, is used as the developer conveying means; and the removed developer is forcibly conveyed to the region of the developer stirring means (1) by attracting the above-described removed developer onto the rotative body of the magnetic conveying means.
As explained above, in the case of a conventional developing unit, after the toner has been supplied onto the latent image formed on an image signal medium, developer in which relatively small amount of toner is contained compared with the amount of carrier, is left on the region of the sleeve (31), so that the concentration of toner left on the sleeve (31) becomes uneven. Consequently, it is necessary to remove all the toner from the sleeve (31), otherwise the image density will undesirably become uneven when an image is formed next time. However, there is a problem that it is difficult to remove all the toner from the surface of the sleeve (31), so that some of the toner will be left on the surface even after toner removal has been conducted.
Such an idea is essentially disadvantageous in that: in order to efficiently convey the developer removed from the surface of the sleeve (31) to the region of the developer stirring means, a magnetic conveying means which is composed of a cylindrical rotative body, on which surface a magnetic pole is formed, is used so that the above-described developer can be attracted onto its surface and the attracted developer is forcibly conveyed to the region of the developer stirring means (1). The reason why this idea is disadvantageous is that the developer attracted to the cylindrical rotative body by magnetic force can not be completely separated from the body when it is conveyed to the position close to the conveyance screw (12). In other words, a portion of the developer is left on the surface of the above-described rotative body and returned to the region where the toner was initially placed.
Another idea has been put forward: after development has been completed, the residual developer, in which the amount of toner is relatively small compared with the amount of carrier, is forcibly removed by a mechanical method. The inventor has already invented a developing device according to the idea described above, and made an application.
However, there is a problem in the developing device in which the residual developer is forcibly removed from the developer holding means by a mechanical method, which is that the removed developer is scattered and leaks outside the container of the developing device through an opening, and as a result, the image formed on an image signal medium is stained.
Refer to FIG. 2b and FIG. 2c.
FIG. 2b is a sectional side view of a developing device relating to another conventional technology, and FIG. 2c is a front view of the developing device, which is taken from its opening.
In FIG. 2c, the circumferential surface of the sleeve (31) corresponding to the axial length of the magnetic means (32), is made coarse, by sandblasting for example. This rugged region is actually a developer holding region, which will be called a sandblasted width in this specification hereinafter.
The numeral (91) in FIG. 2b is an opening provided to one end of the casing means (9). A portion of the above-described developer holding means (3) is protruded from the opening (91) and opposed to an image signal medium not illustrated in the drawing.
The numeral (10) is a developer spill prevention means made of a velvet-like material. One surface of the developer spill prevention means composed of this material is adhered to the inside of the casing means (9) so that the developer spill prevention means (10) can be opposed to the smooth circumferential surfaces of the above-described sleeve (31), wherein the surface of the middle portion of the sleeve (31) is rugged and the surfaces of the edge portions are smooth. The other surfaces of the developer spill prevention means (10) made of velvet-like material are pressed against the smooth circumferential surfaces provided on both sides of the sleeve (31). The function of the developer spill prevention means (10) is as follows: the leakage of developer from the positions around the smooth surfaces on both sides of the sleeve (31) to the outside of the container of the developing device, can be prevented by the developer spill prevention means (10).
However, the above-described developer spill prevention means is pressed against the circumferential surface of the sleeve which composes the developer holding means, so that high frictional resistance is caused when the developer holding means is rotated. Consequently, high torque is necessary to rotate the developer holding means. As a result, the rotation speed of the developer holding means tends to be varied and the density of recorded images becomes uneven. Accordingly, it has been desired to develop a developing device provided with a means which can prevent the leakage of developer from the casing means without using the conventional developer spill prevention means.
Refer to FIG. 2a.
Further, another conventional developing device will be briefly explained as follows.
The rotating members (1), (7) and (3) are generally driven by a single drive means (not illustrated in the drawing). In this case, when the revolution speeds of the rotating members (1), (7) and (3) are varied, the amount of toner supplied to an image signal medium in a unit time is varied, so that uneven density distribution of an image is caused. In order to prevent the unevenness of density, it is common to maintain a constant revolution speed.
In order that uneven image density does not occur and smooth circulation of developer is conducted, there is a developing device comprising: a developer removing means composed of a scraper which removes the developer from the developer holding means (3); and a magnetic conveying means (for example, the magnetic conveying means is composed of a cylindrical rotative body, on the surface of which a magnetic pole is formed) which smoothly conveys the used developer removed from the developer holding means (3) to the region of the developer stirring means (1).
When the developing apparatus according to the above-described idea was realized, it could be confirmed that: the occurrence of uneven image density was effectively prevented; and further the used developer removed from the developer holding means (3) was smoothly conveyed to the region of the magnetic conveying means. However, the following disadvantages were found in this developing unit: the used developer undesirably stays in the region of the magnetic conveying means; and a drive means to drive the magnetic conveying means and the developer holding means (3) could not be smoothly operated due to the residual developer, so that the uneven density distribution was caused in obtained images.
Refer to FIG. 2a.
Further, another conventional developing device will be briefly described as follows.
In the developing device illustrated in FIG. 2a, gaps of about 1 mm are formed between the upper edge (911) of the casing means (9) and the developer holding means (3), and between the lower edge (912) and the developer holding means (3).
There is a disadvantage in the case of the developing device described above, as follows: when the used developer is mechanically removed from the developer holding means (3) by the developer removing means, the removed developer is scattered and leaks outside the container of the developing device, so that the image signal medium and the recording paper become stained.